Estradiol administration to the PMSG primed rat given as a single injection 30 hrs after PMSG reduced follicular atresia from 25.7 + or - 1.7% to 17.1 + or - 1.8% at the 2 mg/kgBW dose. When this dose was given in the form of two injections, one at the time of the PMSG and the other 30 hrs after PMSG, it was even more effective reducing atresia to 7.7 + or - 0.9%. Using the one injection technique the 2 mg/kgBW dose of estradiol was protective against DHT-induced atresia. However, using the two injection technique, even a dose as low as .25 mg/kgBW (approximately 4mug/rat) was effective in preventing atresia (PMSG treated 25.7 + or - 1.3% atretic; PMSG followed by DHT 1 mg/kg BW 41.7 + or - 1.8% atretic; PMSG, two injections of estradiol .25 mg/kgBW and DHT 1 mg/kgBW 29.1 + or - 1.7% atretic). Examining atresia alone, however, was not adequate criteria for preparation of follicle for ovulation. Even though the 0.25 mg/kgBW dose of estardiol appeared to protect the ovary from DHT-induced atresia, less than 50% of the rats ovulated when challenged with hCG. Doses of .5 mg/kgBW of estradiol and higher were protective against DHT-induced atresia even by the criteria of the number of ovulations per rat after hCG. Thus an in vivo test of function is a necessity to supplement any histologic study dealing with follicular atresia. The assay for the measurement of granulosa cell aromatase has been set up. The use of 7 animals (14 ovaries) gives a large enough number of granulosa cells for study even in DHT-treated animals with atretic follicles. The use of 1,2,6,7-tritiated testosterone as substrate permits the use of tritiated water in the incubation medium as a relatively rapid method with a more careful verification using the 3H-estrogen generated as a measure of aromatase. Results using the two approaches have yielded comparable values. In the next year, studies on blood steriods in hypox PMSG-treated rats with or without DHT-induced atresia that have been intiated will be continued to study the effects of DHT on steroidogenesis. Enzymes involved in steroidogenesis, specially 17 alpha-hydroxylase, 5 alpha-reductase and aromatase, will be examined. Receptors for FSH and LH in granulosa cells during DHT-induced atresia will be measured and antiestrogens used to further study the protective effect of estradiol on androgen-induced atresia.